An electrochemical cell includes an anode, a cathode and an electrolyte. Reactants at the anode and cathode react and ions move across the electrolyte while electrons move through an external electrical circuit to form a completed electrochemical reaction across the cell.
An electrochemical cell can be designed to operate only as a fuel cell, where electrical power and heat are output from the electrochemical reaction with fuel and oxidizer as input reactants, or only as an electrolyzer, where input power and reactant (water), and possibly input heat, electrochemically react to produce hydrogen and oxygen, or as a dual purpose electrochemical cell capable of switching between fuel cell and electrolysis modes. Electrolyzer cells operate electrochemically in reverse with respect to fuel cells.
There are multiple types of electrochemical cells which can operate as fuel cells and/or electrolyzers. Some of the most common types of electrochemical fuel cells are proton exchange membranes, solid oxide, molten carbonite, alkaline, and phosphoric acid.
There are many geometries possible for individual electrochemical cells, the two most common types being planar cells and tubular cells. In planar cells the cathode, electrolyte and anode are layered in a planar geometry. In tubular cells the electrolyte is in a tubular configuration with either the anode on the inside of the tube and the cathode external or the cathode on the inside of the tube and the anode external.
Multiple individual electrochemical cells can be configured electrically in series to form a “stack” to match the voltage, power, and current needed for the desired application. For planar electrochemical cell technology, the individual cells are stacked on top of each other with fluid separation plates in between and mechanically fastened together to form the stack of cells electrochemically in series. For tubular electrochemical cell technology, the individual tubes are bundled together with the reactant flow usually shared between the tube inputs and electrical connections at the external outside and ends of the tubes.
Historically, the stack shares a single reactant flow input and output path. The number of cells in the electrochemical cell stack may be selected to provide the cell stack with the desired voltage, current, and power output/input range in response to reactants (i.e., fuel and oxidizer such as hydrogen and oxygen in fuel cell operation, or water and heat in electrolysis operation) passing through the stack.
Stacks may be arranged electrically in parallel or in series with other stacks to support larger power applications. Historically, stacks or subportions of stacks are hardwired electrically into a set configuration.
Historically, when an electrochemical cell stack is supporting a power or load profile (load on a fuel cell or input power to an electrolyzer) that is changing, the entire stack is controlled as a single unit sharing equal load or power production supported by a shared reactant stream. All the cells in the stack historically are designed to operate at the same current density (amperes passing through a set electrochemical cell surface area) to support the power or load profile.
An electrochemical cell operates along a performance curve, commonly called a polarization curve, inherent to each cell. The performance curve dictates how the electrochemical cell voltage changes with the change in current flowing through it.
FIG. 1 is an example proton exchange cell performance or polarization curve operating in fuel cell mode. FIG. 2 is the same example proton exchange cell performance or polarization curve operating in electrolyzer mode. FIG. 3 is an example solid oxide cell performance or polarization curve operating in electrolyzer mode.
In accordance with the electrochemical cell's polarization curve, voltage varies with the change of current density which corresponds to different power levels (input and output).
In a regenerative electrochemical cell's polarization curve, voltage can vary even more significantly between operation in fuel cell and electrolysis mode (reference FIGS. 1 and 2). This difference is compounded as the amount of cells in a stack increase. Thus, a power distribution and control system supporting a single electrochemical cell stack which operates in both fuel cell and electrolysis mode must accommodate a wider range of voltages associated with the electrochemical cell stack.
Historically, to overcome the voltage differences between fuel cell and electrolysis mode in a regenerative electrochemical cell stack, power converters have been used to either boost the output voltage or reduce the input voltage. However, power converters create an efficiency loss, add weight, increase system complexity, and present reliability issues. Another option is to use two separate stacks, one for the fuel cell mode and one for the electrolysis mode. However, additional fuel cell stacks increase the overall weight of the application, present the need for thermal control of the non-operating stack, and introduce reliability issues due to the need for valves that switch between the two modes.
It is with respect to these considerations and others that the disclosure made herein is presented.